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Stock-Based Incentive Compensation Plans - Narrative (Details) - USD ($) | 1 Months Ended | 12 Months Ended | 49 Months Ended
--- | --- | --- | ---
Oct. 31, 2018 | Dec. 31, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Options granted, weighted-average grant date fair value (in dollars per share) |  |  |  | $ 3.98 | $ 1.80 |
Early exercise of stock options, share subject to repurchase (in shares) |  | 16,117 |  | 74,019 | 16,117 |
Liability for early exercise of unvested stock options |  | $ 65,000 |  | $ 331,000 | $ 65,000 |
Forgiveness of notes receivable |  |  |  | $ 0 | 400,000 |
Stock options granted (in shares) |  |  |  | 100,080 |  |
Grant price of stock options granted (in dollars per share) |  |  |  | $ 8.88 |  |
Unrecognized compensation cost |  |  |  | $ 4,500,000 |  |
Unrecognized compensation cost, expected period for recognition (in years) |  |  |  | 2 years 3 months 15 days |  |
Minimum |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Exercise price of option as percent of fair market value when individual owns more than 10% of shares outstanding (percent) |  |  |  | 110.00% |  |
Incentive Stock Options | Minimum |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Exercise price of option as percent of fair market value (percent) |  |  |  | 100.00% |  |
Nonqualified Stock Options | Minimum |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Exercise price of option as percent of fair market value (percent) |  |  |  | 85.00% |  |
Stock Options |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Expiration period (in years) |  |  |  | 10 years |  |
Expiration period when individual holds more than 10% of shares outstanding (in years) |  |  |  | 5 years |  |
Award vesting period (in years) |  |  |  | 4 years |  |
Stock-based compensation expense |  |  |  | $ 2,312,000 | $ 1,875,000 |
Restricted Stock Units | Minimum |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Award vesting period (in years) |  |  |  | 2 years |  |
Restricted Stock Units | Maximum |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Award vesting period (in years) |  |  |  | 4 years |  |
Performance Shares |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Stock options granted (in shares) |  |  | 564,098 |  |  |
Grant price of stock options granted (in dollars per share) |  | $ 4.68 | $ 5.94 |  |  |
Stock-based compensation expense |  |  |  | $ 600,000 |  |
Number of options with modification of terms (in shares) |  | 394,652 |  |  |  |
Performance Shares | Forecast |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Stock-based compensation expense to be recognized related to modification of terms |  |  |  |  |  | $ 800,000
Performance Shares | Vesting Tranche One |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Award vesting period (in years) |  |  | 4 years |  |  |
Stock options granted (in shares) |  |  | 408,544 |  |  |
Performance Shares | Vesting Tranche Two |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Award vesting period (in years) |  |  | 3 years |  |  |
Stock options granted (in shares) |  |  | 155,554 |  |  |
2008 Stock Option Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Number of shares authorized (in shares) |  | 5,350,080 |  |  | 5,350,080 |
2018 Equity Incentive Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Number of shares authorized (in shares) |  |  |  | 2,576,538 |  |
2018 Employee Stock Purchase Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Number of shares authorized (in shares) | 515,307 |  |  |  |  |
Purchase price of common stock as a percent of fair market value (percent) |  |  |  | 85.00% |  |
Employees stock purchase program offering period interval (in months) |  |  |  | 6 months |  |
Stock-based compensation expense | $ 100,000 |  |  |  |  |
Unrecognized compensation cost |  |  |  | $ 300,000 |  |

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